This invention relates to semiconductor laser device and its manufacturing method, especially to those of ridge wave-guide semiconductor laser.
In recent years, the semiconductor laser whose oscillation wavelength is within the range of 600–700 nm is widely used in DVD (digital versatile disc) video player and DVD-ROM (read only memory) drive. Also rewritable and write-once DVD recorder market is rapidly growing and those recorders need higher output semiconductor laser as compared with DVD-ROM. In many laser structures, ridge wave-guide laser is adequate for DVDs.
FIG. 15 is a cross-sectional view of ridge wave-guide semiconductor laser device. This semiconductor laser has a double hetero-junction composed of InGaAlP formed on a GaAs substrate, an upper cladding layer having a ridge shape thereon and a current blocking layer on both sides of the ridge shape cladding layer. Since the current blocking layer is transparent at oscillation wavelength, this structure is called “real refractive-index wave guide”.
Manufacturing process of this semiconductor laser is explained as follows.
First, an n-type cladding layer 2 of In0.5(Ga0.3Al0.7)0.5P, a guide layer 3 of In0.5(Ga0.5Al0.5)0.5P, an MQW active layer 4 of InGaP/InGaAlP, a guide layer 5 of In0.5(Ga0.5Al0.5)0.5P, a first cladding layer 6 of In0.5(Ga0.3Al0.7)0.5P, a p-type etching stop layer of In0.5Ga0.5P, a p-type second cladding layer 8 of In0.5(Ga0.3Al0.7)0.5P, and a p-type intermediate layer 9 of In0.5Ga0.5P are grown sequentially on the n-type GaAs substrate 1.
Subsequently by etching the intermediate layer 9 and the second cladding layer 8 vertically using RIE (reactive ion etching), a ridge stripe 10 is formed. An n-type current blocking layer 11 of In0.5Al0.5P is grown on both sides of the ridge stripe 10, and a p-type GaAs contact layer 12 is grown thereon. Finally, an n-side electrode 13 is formed on a backside of the substrate 1, and then a p-side electrode 14 is formed on the p-type GaAs contact layer 12.
This ridge stripe lasers has advantages to control the stripe width strictly and hence to obtain the excellent laser performance because the vertical sidewalls are formed by anisotropic RIE etching technique. Such a ridge stripe laser is disclosed in PCT international publication number WO 00/021169 A1.
However, there occur problems explained below in such ridge stripe laser.
FIG. 16 is a cross-sectional view near the lower corner edge of the ridge stripe. When the n-type current blocking layer 11 of In0.5Al0.5P is grown on the cladding layer 6 and the sidewalls of ridge stripe 10, the crystal grows perpendicularly to both surfaces, respectively, as shown with arrows G1 and G2. Because crystal growth directions G1 and G2 are approximately perpendicular to each other, the growth fronts collide near the lower corner edge of the ridge stripe 10. In this collision region, the growth surface morphology becomes poor and hence crystal defects increase. This defect-rich region “D” is shown in FIG. 16 by hatching.
Since the defect-rich region D includes a lot of crystal defects, current blocking effect becomes imperfect. Therefore, the leak current and the threshold current increase.
One example of the defect-rich region is explained hereafter. In a case of MOCVD (metal-organic chemical vapor deposition) process, material gases are not supplied sufficiently near the lower corner edge of the ridge stripe 10. Therefore, an imperfect layer which contains the voids is grown in the current blocking layer 11, as shown in FIG. 17.
The leak current increases if such voids are formed in the current blocking layer 11. Also because an optical absorptive GaAs contact layer 12 penetrates into the voids near a light emitting region, an optical loss occurs and causes the lower light emitting efficiency.